Nowadays, the SCSI over PCI Express (SOP) standard, obtained by mapping the SCSI standard over PCI Express (Registered Trademark, hereinafter referred to as “PCIe”), has been presented as interface between a host and a data storage apparatus (storage device).
As a characteristic of the interface standard like this, a plurality of queues configured to queue request information to request processing for a command or a task and response information from the storage device are allocated on the host side. In addition, an interface control specification for controlling the queues on the storage device side is implemented.
The request information and response information which are stored in the queues are also referred to as information units (IUs), and referred to as request IUs and response IUs, respectively. The information unit may be stored as one element, or divided into a plurality of elements and stored in the queue. The term “element” indicates a storage unit of the queues which store the information units.
When request information which are divided into a plurality of elements and stored in the queue is issued to the storage device (specifically, when the storage device fetches the request information), part of the divided elements may stay in the queue when the request information is transmitted by unit of element. When such a situation occurs, the storage device cannot process the request information until all the elements are transmitted, and thus the processing efficiency decreases.